1. Field of Invention
This invention relates generally to anemometer and generalized sensor fouling and, more particularly, to antifouling of a micro-anemometer.
2. Background Art
An anemometer is a device that is used for measuring wind speed, and fluid flow. Hot-wire anemometers can measure the wind's velocity. There is a close connection between the pressure and the velocity, thus, an anemometer designed for one can give information about both when accompanied by a collocated pressure sensor. One type of anemometer is a Hot Wire anemometer, which uses a very fine wire (on the order of several micrometers) electrically heated up to a predefined temperature above the ambient. Air flowing past the wire has a cooling effect on the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the flow velocity. Hot-wire devices can be further classified as CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA (Constant-Temperature Anemometer). The voltage output from these anemometers is thus the result of some sort of circuit within the device trying to maintain the specific variable (current, voltage or temperature) constant. Additionally, PWM (pulse-width modulation) anemometers are also used, wherein the velocity is inferred by the time length of a repeating pulse of current that brings the wire up to a specified resistance and then stops until a threshold “floor” is reached, at which time the pulse is sent again.
Hot-wire anemometers having a very fine structure can have a very high frequency-response and fine spatial resolution compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest. However, a downside to the very fine structure is the problem with fouling. If debris accumulates on the very fine structure, it can significantly affect measurement efficacy of the device.
Hot-wire anemometers are often used to measure fluid velocity and wind speed based on the amount of heat connected away by a fluid or wind passing over a heated wire. In typical hot-wire anemometers, a hot wire or filament is heated by either a constant current (constant-current anemometers) or, alternatively, heated to a constant temperature (constant-temperature anemometers). In either case, the amount of heat lost due to convection is a function of the fluid or wind velocity passing over the filament.
The amount of heat that is dissipated by a heated filament located in a fluid stream depends on a number of factors including the filament's temperature, the geometry of the filament, the temperature, density and type of the fluid, and the fluid velocity. The filament's temperature is determined by measuring its electrical resistance. Empirical data and/or mathematical algorithms are used to calculate the temperature and the flow rate based on the measured resistance. Because metals used to fabricate suitable filaments have resistivity coefficients on the order of 0.1%/.degree. C., a high degree of accuracy is needed for measuring the actual resistance of the filament. However, due to the fact that the filament is exposed to the wind or fluid passing over it, any debris flowing along with the wind or fluid can cause fouling of the wire or an accumulation of debris on the surface of the wire. Again, due to the very fine structure, not much debris is required to negative effect the efficacy of the anemometer.
Measuring flow velocity of fluids and wind can be very helpful in a variety of applications, including for example meteorology or oceanography research, metering, monitoring, and similar applications. Hot-wire anemometers have also proven useful for macroscale measurements. For example, hot-wire anemometers are used to monitor air flow within automobile engines, ventilation and heating ducts, and the like. However, due to the nature of this type of research and the type of debris that can be present and the fine structure of a hot-wire anemometer, which is positioned within the flow of media, such as gas, liquid, particle-laden liquid, or the like, then fouling can be a significant problem require regular maintenance. As the media flows past the hot wire, not only is heat transferred from the hot wire to the media, thus cooling the hot wire, but in addition the wire is typically fouled by debris. Flow can't be accurately determined from the temperature variation effects on the hot wire because the accumulated debris will affect the temperature variation.
Historically, conventional macroscale hot-wire anemometers could not easily be scaled into microscale dimensions. For example, as the scale of components was reduced, the components became fragile and be easily damaged during the fabrication process. However, micro and nano-anemometers are now being developed, thus there is a need to resolve the fouling issue.
Again, as mentioned above, operation of the hot wire nano-anemometer occurs when media is introduced into the open space between the electrodes so it flows past and envelops the nanowire. The nanowire may be heated, for example, by passing an electrical current through the nanowire. As the media flows past the nanowire, it will remove heat from the nanowire. Flow rate of the media can be inferred by measuring the effects of this heat loss. One technique is to measure a temperature decrease of the nanowire resulting from the media flow using the sensing apparatus. For higher flow rates, greater temperature decreases will occur. Alternately, the nanowire can be held at a constant voltage or current by the sensing apparatus, and resistance changes due to temperature changes determined from which the flow rate can be inferred. As yet another example, the nanowire may be held at an approximately constant temperature by varying current (or voltage), and the changes in current (or voltage), and the flow rate inferred from the variations in current (or voltage). However, each of these measurement techniques can be negatively impacted by fouling.
Various units of measure for flow rate can be used. For example, flow rate may be expressed as a velocity (e.g. distance/time), mass times velocity, mass per unit time, volume per unit time, etc. For example, from a flow velocity measurement, flow volume rate can be calculated by taking into account the dimensions of the open area through which the media is flowing. Various other measures, such as total flow over a given time interval, can also be obtained, as will be appreciated. While high resolution measurements of flow rate or flow volume can be performed, coarse measurements can also be performed.
The nanowire can be a material for which the temperature of the nanowire affects an electrical property of the nanowire. For example, the nanowire can be formed from conductive material, semiconductor material, or doped semiconductor material. However, fouling can affect the conductivity characteristics of the wire. Various semiconductors can be used, including for example silicon, germanium, indium phosphide, other III-V compounds, II-VI compounds, etc. As a particular example, the resistance of silicon and silicon compounds can be strongly dependent upon temperature. Hence, the resistance of a silicon-containing nanowire can be measured to obtain information about the temperature of the nanowire. The nanowire can be heated by applying a current flow through the nanowire. Cooling of the nanowire by the flowing media past the nanowire will cause a change in the resistance of the nanowire. However, in each of these scenarios the measurement accuracy is impacted by fouling.
In addition to adversely altering the performance of hot-wire anemometers, other types of precision sensors are also adversely affected by atmospheric foulants. These sensors include microelectromechanical systems (MEMS) and mirrors which operate on many wavelengths. Foulants that form or adhere to the surfaces of such sensors can significantly corrupt the signals. These foulants include all particulate matter from dust to mites, dirt, debris, mold, mold spores, water, water droplets, ice and even local water condensate.